A source tells The New York Daily News an investigator was paid for by a celebrity magazine to follow Kevin Federline to investigate how close to certain Las Vegas call girls the struggling rapper had been hanging out with before wife Britney Spears filed for divorce. “Prostitution is legal in Nevada, so it’s no big deal to say he was ‘partying’ with them at a club,” one night crawler said. “Also, there’s no way to prove he even knows what they do for a living.”